As a network file system, Network attached storage (NAS) is known. In recent years, a system that employs a mixture of storage devices with various access characteristics in addition to NAS has also been proposed.
Examples of storage devices with a variety of access characteristics that may be mentioned include Write Once Read Many (WORM) devices, tape devices, and a Massive Array of Idle Disks (MAID). In the case of a WORM device, update processing is not allowed after initial write processing. Hence, data that has been written to a WORM device cannot be deleted or falsified. In the case of MAID, power consumption is reduced by stopping or decreasing disk rotation in the period of no disk access.
Expectations for hierarchical file systems have risen. For instance, by making combined use of a plurality of such storage devices with different access characteristics, files are stored in an optimum storage device according to the importance or usage characteristics of the files (Patent Literature 1).